GB2349553A

GB2349553A - Transmission power control in a radio communication system

Info

Publication number: GB2349553A
Application number: GB0010325A
Authority: GB
Inventors: Mark Wentworth Rayne; Diana Margaret Ball
Original assignee: Simoco International Ltd
Current assignee: Simoco International Ltd
Priority date: 1999-04-27
Filing date: 2000-04-27
Publication date: 2000-11-01
Anticipated expiration: 2020-04-27
Also published as: AU4586000A; GB9909688D0; WO2000065746A1; GB0010325D0; GB2349553B

Abstract

In a mobile radio communications system, which includes a radio cell served by colocated base stations BS2 and BS3, Each mobile radio unit MS4, MS5, MS6, stores a desired power level limit for the reception of signals at the base stations of the radio system. Each mobile unit also stores a predetermined transmit power level representative of the transmit power level used by the base stations. The path loss from base station to mobile station may be determined from the received power level and the predetermined transmit power level. The mobile radio units use these values along with the desired power level for reception of signals at the base stations to derive a transmit power level for their own transmissions. This avoids transmissions reaching a base station at too high or low a signal level.

Description

2349553 Radio Communications System The present invention relates to radio

communications systems and in particular to a method of and an apparatus for controlling the power level at which mobile radio units transmit in a mobile radio communications system.

Most mobile radio communications systems comprise one or more fixed radio base stations and plural mobile radio units which are capable of radio communication with the fixed base stations. The area covered by each base station or co-located group of base stations is generally known as a cell.

As is known in the art, the power level at which a radio signal transmitted by a mobile radio unit is received at a base station will depend on the proximity of the mobile radio unit to the base station. This means that in practical radio communications systems the received power level at a base station for a signal of a given transmit power can vary by a factor of up to 110dB (a ratio of 10") depending on whether the mobile radio unit is very close to the base station or at the edge of the cell served by the base station. It is desirable for base stations to be able to cope with this received signal power range.

However, there are practical difficulties in designing suitably sensitive radio receivers which can also operate reliably when presented with very strong signals (such as might be the case from nearby mobile radio units) Also, a relatively strong signal from a close mobile radio unit can lead to reception difficulties at a base station where signals from plural mobile radio units are being received by the base station. For example, a relatively strong signal from a nearby mobile radio unit can interfere with a weak signal on a different radio channel (as the sideband from the local, stronger transmitter may be stronger than the weaker, wanted signal) therefore making reception of that weaker signal more difficult. Also, in a time-division multiple-access (TDMA) system a strong signal received by a base station on a first timeslot might cause a large DC offset in the receiver which the base station receiver may not be able to remove in time to receive properly a weak signal from another mobile station in the next timeslot (as, e.g. the weak signal could be less than the DC offset caused by the large signal).

Thus it is desirable to be able to control the power level at which mobile radio units transmit to base stations in a mobile radio communications system to reduce the occurrence of overly strong signals that could lead to the above problems.

In one known method of controlling the power level at which mobile radio units transmit to base stations in a mobile radio communications system, the base stations of the system control the transmit powers of the mobile radio units by transmitting appropriate power control messages addressed to the mobile radio units. This power control method is often known as closed loop power control and is used in, inter alia, the TETRA (TErrestrial Trunked RAdio) and GSM (Global System for Mobile communication) systems. A drawback of this power control method is that it can only be used af ter the mobile radio units have transmitted to and established communication with the base station (so that the base station can assess the received power of the mobile radio unit Is signal). Thus it cannot, for example, be used to control the power at which mobile radio units make their initial access call request attempts to a base station.

In a second known power control method, of ten referred to as open loop power control, each base station broadcasts a message indicating its transmission power level and desired received signal power level.

Mobile stations wishing to contact a base station measure the strength of the signal that they receive from the base station and using the transmission power level parameter transmitted by the base station, derive an estimated power loss for the transmission path between the base station and the mobile radio unit. The mobile radio unit then uses that estimated power loss to attempt to adjust its transmitter power level so that its signals reach the base station at the power level specified by the received signal power level parameter transmitted by the base station. An advantage of this method is that it does not require the mobile unit to have transmitted to and established radio communication with the base station before it can be used. Thus it can be used to control the mobile station's transmitted power for initial access attempts to a base station.

However, it does require base stations to transmit information relating to their transmit power levels, etc. to the mobile radio units.

According to a first aspect of the present invention, there is provided a method of controlling the power level at which a f irst radio unit of a radio communications system transmits to another radio unit of the system, the method comprising:

determining the power level received at the f irst radio unit of a radio signal from the other radio unit; using the determined received power level, a first stored predetermined power level value representative of the transmit power level of the other radio unit, and a second power level value representative of a desired received power level at the other radio unit, to derive a transmit power level for the first radio unit; and controlling the first radio unit to transmit signals to the other radio unit at a transmitted power level which is the lower of the derived transmit power level and a defined maximum transmit power level of the first radio unit.

According to a second aspect of the present invention, there is provided an apparatus for controlling a radio unit of a radio communications system, the apparatus comprising:

means for storing a first predetermined power level value representative of the transmit power level of another radio unit of the radio communications system; means for storing a second power level value representative of a desired received power level of another radio unit of the radio communications system; means for estimating the power level received at the radio unit of a radio signal from another radio unit of the radio communications system; means for using the estimated received power level, and the first and second stored power level values, to derive a transmit power level for the radio unit; means for comparing the derived transmit power level and a defined maximum transmit power level for the radio unit; and means for controlling the radio unit to transmit its signals to the other radio unit at the lower of the derived transmit power level and the defined maximum transmit power level for the radio unit.

In the present invention the transmission power level of a radio unit of a radio communications system (e.g. of a mobile radio unit of a mobile radio communications system) is controlled to be the lower of a derived transmit power level and a defined maximum transmit power level for the radio unit. This helps to ensure that the radio unit's transmission is received at the receiving radio unit (e.g. base station of a mobile radio communications system) below a given power level (i.e. helps to protect the receiving radio unit from overload).

The derived transmit power level is, in contrast to, for example, open loop power control, where the actual contemporaneous base station transmit power level is used, derived in the present invention using, inter alia, a stored pre- assigned predetermined transmit power level value for a given radio unit (e.g. base station) of the radio communications system. The Applicants have found that, for example, a stored predetermined transmit power level value for a base station of a mobile radio communications system can be used satisfactorily to ensure that a mobile radio unit's signal is received at a base station below a given power level.

The present invention therefore does not require any power level parameter transmission from radio units, such as base stations, and can therefore avoid the need for radio units, such as the base stations, of the system to expend air time broadcasting their transmission power level (as is necessary, for example, in open loop power control) or broadcasting a transmission control parameter to the radio unit (as is necessary in closed loop power control). This, for example, simplifies base station operation and also means that the present invention is applicable to radio systems which do not transmit an indication of the base station's transmission power, etc. such as the MPT1327 system.

There is also no need when performing power control in accordance with the present invention for the first, transmitting radio unit to have first communicated with the other radio unit, e.g. base station (unlike closed loop power control).

The present invention is particularly applicable to controlling the power level at which mobile radio units of a mobile radio communications system transmit to other radio units, e.g. in particular base stations, but also other mobile radio units of the system. Thus in a particularly preferred embodiment the first radio unit is a mobile radio unit of a mobile radio communications system and the other radio unit is a base station or another mobile radio unit of the system.

6 - Thus according to a third aspect of the present invention, there is provided an apparatus for controlling a mobile radio unit of a mobile radio communications system, the apparatus comprising:

means for storing a first predetermined power level value representative of the transmit power level of a base station of the mobile radio communications system; means for storing a second power level value representative of a desired received power level of a base station of the mobile radio communications system; means for estimating the power level received at the mobile radio unit of a radio signal from a base station of the mobile radio communications system; means for using the estimated received power level, is and the first and second stored power level values, to derive a transmit power level for the mobile radio unit; means for comparing the derived transmit power level and a defined maximum transmit power level of the mobile radio unit; and means for controlling the mobile radio unit to transmit its signals to the base station at the lower of the derived transmit power level and the defined maximum transmit power level for the mobile radio unit.

The invention is, for example, also applicable to controlling the power at which mobile radio units of a mobile radio communications system transmit directly to each other (i.e. independently of a fixed radio network and base stations), such as mobile radio units operating in the TETRA Direct Mode. In this case both the f irst and other radio units would be mobile radio units. The invention is accordingly also applicable to controlling the power level at which TETRA mobile radio units transmit to Direct Mode repeaters and Direct Mode gateways.

The estimated received signal level and first and second power level values can be used as desired to derive a transmit power level for the radio unit.

In a preferred embodiment the path loss for the signal from the other radio unit, e.g. base station (or Direct Mode mobile radio unit) to the first radio unit, e.g. mobile radio unit, is estimated, for example by determining the difference between the stored predetermined radio unit (e. g. base station) transmission power level value and the estimated received signal:

Ld = Pb. - Pmi where Ld is the estimated path loss from the other radio unit (e.g. base station) to the first radio unit (e.g. mobile radio unit, i.e. on the downlink), Pb, is the is predetermined stored (i.e. assumed) output power of the other radio unit (e.g. base station), and Pmj is the average power received by the first radio unit.

The estimated path loss can then, for example, be summed with the stored power level value representing the desired received signal level at the other radio unit (e.g. base station) to derive first a radio unit transmit power level that should arrive at the radio unit at the desired level.

The derived transmit power level can then be used as an upper limit or maximum for the radio unit's transmit power level, so as to ensure that its transmission is received at the other radio unit (e.g. base station) below a given maximum power level. Thus the transmission output power, P,,, of the first (e.g.

mobile) radio unit is then controlled to be the lower of a defined or stored maximum output power level, Pm,,,,, and the derived radio unit transmit power level:

PM,, = Min (Pmax, Pbi + Ld - e) where Pbi is the desired received signal level at the other radio unit (e. g. base station), Ld is the estimated path loss, and e is an optional error margin to allow for errors in the estimate of the path loss, Ld, and errors in the setting of the radio unit's transmit power to PM,.

It can be seen that the above calculation of the transmit power level for the transmitting, first radio unit is similar to the calculation used in the open loop power control method, but the present invention uses a stored predetermined transmit power level for the other radio unit (e.g. base station) rather than a contemporaneous, actual power level value transmitted by the other radio unit to perform the power level calculation.

If desired, a second calculation to determine the is minimum possible power level for transmission to the other radio unit, e.g. base station may be performed:

PmO, (Pmax f Pbmill + L,, + e) where Pbmin is the minimum receive level acceptable to the other radio unit e.g. base station. The transmission output power of the first radio unit e.g. mobile unit, can then be controlled to be the higher of the desired receive signal level, Pm, at the other e.g.

base station unit, and the minimum receive level, PmO. at that unit.

Pm,., = max (Pmo r PMO,) By performing this calculation, it can be ensured that the power is not turned down so low that the transmission by the first e.g. mobile radio unit is not received, although this may be at the risk of overloading the other radio unit, e.g. base station.

The defined maximum transmission power level, Pm,,x, of the first, transmitting radio unit (e.g. mobile radio unit) could simply be the maximum power level at which the radio unit is capable of transmitting (i.e. its transmitter's maximum power level). In addition to this, the radio system may have defined maximum permitted transmit power levels for any radio unit in a particular situation, such as in a particular location (e.g. cell). For example, in a mobile radio communications system there may be maximum transmit power levels defined for particular or all cells of the system. These maximum permitted transmit power levels could, for example, be predetermined, and may be prestored in the radio unit. Alternatively, they could be broadcast to the radio unit by the radio system (and then stored by the radio unit) In this case the defined maximum transmit power level that will be compared with the derived transmit power level will be the lower of the maximum transmit power level currently permitted by the system for the radio unit (e.g. in the radio unit I s current cell) and the maximum power level at which the radio unit is actually capable of transmitting.

The received power level of the e.g. base station's signal at the e.g. mobile radio unit can be estimated as desired, e.g. by averaging the received signal strength over a time period of, for example, a few seconds. For example, in the TETRA system, the received power level could be estimated by averaging the output value of the receiver's RSSI (Received Signal Strength Indication) detector over a period of a few seconds and multiplying the result by a predetermined calibration value.

Alternatively, the peak received signal in a predetermined time period could be used as the estimated received signal power.

The stored predetermined transmit power level value indicative of the transmit power of the other radio unit, e.g. base station, can be selected as desired. The radio unit e.g. mobile unit could store a table of transmit power levels for many other radio units e.g.

base stations, e.g. levels suitable for each cell. Preferably, however, the radio unit or apparatus stores and uses a single such value to be used as being representative of the transmit power level of plural radio units of the system, preferably of at least of all radio units of the radio communications system of a particular type. Thus a single predetermined transmit power level value could be stored as representing the transmit power level of all base stations of the system.

Preferably just one predetermined transmit power level is stored. Preferably, plural radio units, e.g. preferably each radio unit at least of a particular type, e.g. each mobile radio unit, stores and uses the same predetermined transmit power level value or values.

The predetermined transmit power level value could, for example, be stored in the radio unit or apparatus before it is used or has communicated with another radio unit, e.g. base, station and independently of any radio communication with another, e.g. base station of the system.

The stored predetermined other radio unit transmit power level value is preferably based on and most preferably corresponds to one of the lowest, and most preferably substantially the minimum, possible transmit power level usable by any radio unit of that type. Thus, where the radio system is a mobile radio communications system and the other radio unit is a base station of that system, the stored predetermined base station transmit power level value is preferably based on and most preferably corresponds to one of the lowest, and most preferably substantially the minimum, possible transmit power level usable by any base station of the system, i.e. such that the mobile unit effectively assumes for the purposes of power control that all the base stations of the system are operating substantially at the lowest possible power level. Thus, for example, where individual base stations can transmit at different power levels and/or different base stations of the system can transmit at different power levels, the or each mobile radio unit or apparatus stores and uses one of the lowest such values and preferably substantially the lowest such value as its predetermined base station transmission power level value.

The effect of this arrangement is that if the base station is transmitting at the stored predetermined power level, the mobile radio unit or apparatus will derive an appropriate transmission power level. If the base station is transmitting at a higher power level, the mobile radio unit or apparatus will tend to underestimate the path loss to the base stations, such that the mobile radio unit's transmission will be received by the base station at a power level below the level predicted by the mobile radio unit. The system will normally still function adequately under such conditions. (On the otherhand, if a higher predetermined power level value was used, then if a base station were to transmit at a power level lower than the predetermined value, the mobile unit would overestimate the path loss, leading to a risk of the mobile unit transmitting at a level that would arrive at the base station at a higher level than desired.) Indeed it is believed that this arrangement is advantageous in its own right. Thus, according to a fourth aspect of the present invention, there is provided a method of controlling the power level at which a radio unit of a radio communications system transmits to a base station of the system, the method comprising:

determining the power level received at the radio unit of a radio signal from the other radio unit, using the determined received power level, a first stored predetermined power level value corresponding to a predetermined minimum possible transmit power level of the other radio unit of the system, and a second power 12 - level value representative of a desired received power level at the other radio unit, to derive a transmit power level for the radio unit; and controlling the radio unit to transmit signals to the other radio unit at a transmitted power level which is the lower of the derived transmit power level and a defined maximum transmit power level for the radio unit.

In this arrangement, the transmitting radio unit whose power is being controlled could, for example, be a mobile radio unit and other radio unit could, for example, be a base station, of a mobile radio communications system.

The stored power level representative of the desired received power level at the other radio unit (e.g. base station) can also be selected as desired. This value is also preferably predetermined, and, preferably pre-assigned for a given radio unit or units of the system, and can be stored prior to use of the radio unit, and prior to any communication with another radio unit and independently of any radio communication with another radio unit of the system. Again, the and preferably each radio unit or apparatus preferably stores and uses a single such value to be used for plural radio units, and preferably for all the radio units of a particular type, e.g. base stations, of the system.

In a particularly preferred embodiment of the present invention, which is particularly applicable to controlling the power level at which mobile radio units transmit to base stations of a mobile radio communications system, the derived transmit power level is the power level considered necessary by the transmitting (e.g. mobile) radio unit for its signal to arrive at the other radio unit (e.g. base station) close to (e.g. within a predetermined margin below), and most preferably at, but not above, the maximum permitted received signal level at the other radio unit (e.g. base station) for signals from a radio unit of the type of the first transmitting radio unit (e.g. a mobile radio unit). In this arrangement the e.g. mobile radio units therefore effectively try to ensure that their signals arrive at the e.g. base station at as high a power level as possible but which level is not above the maximum permitted received power level at the base station (although the mobile radio unit will, as discussed above, tend to underestimate its necessary power level if the base station is transmitting at a power level higher than the predetermined base station transmit power level stored and used by the mobile radio unit).

To achieve this the stored value of the desired received power level at the e.g. base station can be based on and preferably is the maximum power level at which the system permits signals to be received at the base station (and preferably at any base station of the system (or the lowest of the maximum levels if different base stations have different maximum permitted received power levels)). The e.g. mobile radio unit can then use this figure and e.g. the calculated path loss to derive a transmit power level at which it would expect its signal to arrive at the base station at the maximum permitted level (or within a predetermined margin below it) (although as noted above its calculated value may not in practice arrive at the expected value if the base station is transmitting at a different power level to the stored value used by the mobile radio unit in its calculations).

This embodiment encourages, for example, mobile radio units to transmit to a base station at their defined maximum permitted power level (since they will tend to calculate that an even higher power level is needed to transmit to the base station to arrive at the desired high maximum level). Thus in this embodiment in practice many mobile radio units even operating the power control method of the present invention will tend to transmit to the base station at the same power level, i.e. their defined maximum permitted power level, even if they are at different distances (and therefore path losses) from the base station. The signals from these mobile radio units transmitting at their defined maximum power level will tend to arrive at the base station over a range of power level values (because the mobile units will tend to have different path losses to the base station).

This contrasts with, for example, the usual open loop power control arrangement which is generally arranged so that the base station receives all signals at a level just above the base station's limiting receiver sensitivity (after taking into account fading margins and measurement errors), i.e. such that all mobile units' signals are intended to arrive at the base station at the same low (i.e. minimum possible) power level. In this open loop power control arrangement the mobile radio units will tend to all transmit at their derived power level (rather than their maximum power level) and thus their signals will all tend to arrive at the base station at the same or a similar relatively low level, rather than over a range of values as tends to be the case with this embodiment of the present invention.

However, the Applicants have recognised that it can be advantageous to have signals from mobile radio units arrive over a range of power levels at a base station.

As is known in the art, access attempts to base stations in a radio system, and in particular trunked radio systems, often require mobile radio units wishing to make a call to transmit a call set up request to the base station on a special control radio channel. The times when the individual mobile units make call requests on the control channel are often randomised to a certain degree, e.g. using some form of aloha, control mechanism, to try to avoid call requests from different mobile units arriving simultaneously and - therefore colliding. Nonetheless collisions do occur and in that event a significantly stronger mobile radio unit signal will capture' the base station's receiver, i.e. it will be received and the other access attempts will not be received (such that those mobile radio units will have to make further access attempts until they are successfully received). However, at least one signal will usually be successfully received and so the number of mobile radio units making access attempts will gradually reduce until all are successful.

If, as is the case for open loop power control, all mobile radio units are received at the base station at the same or similar power level, the likelihood of there being a stronger signal that will capture the receiver when collisions occur is reduced, thus increasing the chances that all mobile units will have to try again to access the base station after a collision. This increases access time delays and the likelihood of traffic channels remaining unallocated, and reduces the spectral efficiency of the system.

On the otherhand, where there is less compression of the range of received power levels at the base station, as in the present embodiment of the present invention, there will be better operation of the aloha 'capture effect'. Thus the present invention in this embodiment is less detrimental to efficient operation of the radio system than open loop power control, but still ensures that the mobile unit's signals arrive at a sufficiently low level so as not to overload the base station.

Thus according to a fifth aspect of the present invention, there is provided a method of controlling the power level at which a mobile radio unit of a mobile radio communications system transmits to a base station of the system, comprising:

deriving a transmit power level for the mobile radio unit on the basis of the received power level of a signal from a base station of the system and a defined maximum permitted received signal level at the base station; and controlling the mobile radio unit to transmit to the base station at the lower of the derived transmit power level and a defined maximum transmit power level for the mobile radio unit; wherein the derived transmit power level is the power level which is estimated to be necessary for a transmission from the mobile radio unit to arrive at the base station ator within a predetermined margin below the defined maximum permitted received signal level.

In this embodiment, the derived transmit power level could be derived by estimating the path loss to the base station by subtracting the received power level of the base station's signal from an, e.g. predetermined, power level value representing the transmit power level of the base station, and then adding the estimated path loss to the maximum permitted received signal level at the base station (optionally subtracting an error margin from the resulting value) to arrive at the derived transmission level value.

Where the present invention is used in a mobile radio communications system, each mobile radio unit of the mobile radio communications system preferably stores and uses the same predetermined values (or preferably single value) indicative of the transmit power of the base stations and the same predetermined values (or preferably single value) indicative of the desired received power level at the base stations of the mobile radio communications system, although this is not essential and different mobile radio units and apparatus could store and use different values, if desired.

Thus according to a sixth aspect of the present invention, there is provided a mobile radio communications system comprising plural mobile radio units and plural base stations capable of radio communication with the mobile radio units, wherein each mobile radio unit comprises:

means for storing a first predetermined power level value to be used as representing the transmit power level of each base station of the mobile radio communications system; means for storing a second predetermined power level value to be used as representing the desired received power level of each base station of the mobile radio communications system; means for estimating the power level received at the mobile radio unit of a radio signal from a base station of the mobile radio communications system; means for using the estimated received power level, is and the first and second stored power level values, to derive a transmit power level for the mobile radio unit; means for comparing the derived transmit power level and a defined maximum transmit power level for the mobile radio unit; and means for controlling the mobile radio unit to transmit its signals to the base station at the lower of the derived transmit power level and the defined maximum transmit power level for the mobile radio unit; wherein each mobile radio unit of the system stores and uses the same first and second predetermined power level values.

In a particularly preferred embodiment of the present invention a transmission power level for the first, transmitting radio unit is only derived using the stored predetermined power level value if the estimated received signal power from the other radio unit (e.g.

base station) is above a predetermined threshold level.

If the estimated received signal power is below the threshold level, the radio unit then simply uses its maximum power level (i.e. the lower of the maximum power level its transmitter is capable of and the maximum power level currently permitted for it by the radio 18 - system (if set) or another defined power level, for its transmission. This arrangement has the advantage of avoiding the need to perf orm a path loss (or other) calculation, thereby saving processing time, when the other radio unit's (e.g. base station's) signal is received below the threshold level.

The threshold could therefore, for example, be set at the level which indicates a path loss that would tend to cause a signal at the radio unit's defined maximum transmission power level to arrive at less than the maximum received power level of the other radio unit's receiver. The threshold could vary in accordance with the varying maximum transmitted power levels permitted by the radio system e.g. in particular or all cells of the system, or be set differently for e.g. transmissions to base stations and for transmissions to mobile radio units.

It is believed that this arrangement is advantageous in its own right. Thus, according to a seventh aspect of the present invention, there is provided a method of controlling the power level at which a first radio unit of a radio communications system transmits to another radio unit of the system, the method comprising:

estimating the received power level at the first radio unit of a signal from another radio unit of the radio communications system; determining whether the received power level is above a predetermined threshold level; if the received power level is above the predetermined threshold level, deriving a transmit power level for the first radio unit based on the received power level; and if the received power level is not above the 3S predetermined threshold level controlling the radio unit to transmit signals to the other radio unit at a defined transmit power level.

According to an eighth aspect of the present invention, there is provided a radio unit for use in a radio communications system, the radio unit comprising:

means for estimating the received power level at the radio unit of a signal from another radio unit of the radio communications system; means for determining whether the received power level is above a predetermined threshold level; means for, if the received power level is above the predetermined threshold level, deriving a transmit power level for the radio unit based on the received power level; and means for, if the received power level is not above the predetermined threshold level, controlling the radio unit to transmit signals to the other radio unit at a defined transmit power level.

Another particular advantage of this arrangement is that only those radio units receiving signals above the threshold level (e.g. those mobile radio units close to base stations) actively control their transmission power levels to be below the maximum radio unit transmission power level. Thus, for example, in practice most of the mobile radio units in the cell will tend to transmit at their defined maximum power level, regardless of their distance from the base station. Due to the different path losses from these mobile radio units their signals will be received at different power levels at the base station, thereby, as discussed above, facilitating the aloha "capture effect" (unlike in the open loop power control method where all the mobile radio units, signals tend to arrive at the base station at the same or similar power levels, regardless of their distance from the base station).

Thus in this preferred embodiment, the present invention will protect base station receivers from too high signals, but tend to cause less damage to the aloha mechanism because only the mobile units closest to a base station reduce their power levels.

Thus, according to a ninth aspect of the present invention, there is provided a method of operating a radio unit of a radio communications system, the method comprising:

estimating the received power level at the radio unit of a signal from another radio unit of the radio communications system; determining whether the received power level is above a predetermined threshold level; if the received power level is above the predetermined threshold level, controlling the radio unit to transmit signals to the other radio unit at a power level lower than a defined maximum transmit power level of the radio unit; and if the received power level is not above the predetermined threshold level, controlling the radio unit to transmit signals to the other radio unit at the defined maximum transmit power level of the radio unit.

According to a tenth aspect of the present invention, there is provided a mobile radio unit for use in a mobile radio communications system, the mobile radio unit comprising:

means for estimating the received power level at the mobile radio unit of a signal from a base station of the mobile radio communications system; means for determining whether the received power level is above a predetermined threshold level; means for, if the received power level is above the predetermined threshold level, controlling the mobile radio unit to transmit signals to the base station at a power level lower than a defined maximum transmit power level of the mobile radio unit; and means for, if the received power level is not above the predetermined threshold level, controlling the mobile radio unit to transmit signals to the base station at the defined maximum transmit power level of the mobile radio unit.

In the above eighth and ninth aspects of the present invention, the transmit power level of the radio unit can be controlled to be less than the maximum transmit power level of the radio unit as desired when it is necessary to do so (i.e. the received power level is above the predetermined threshold level) For example, the radio unit could be controlled to use a predetermined power level that is less than its defined maximum level, or to reduce its current (e.g. maximum) transmit power by a predetermined amount (e.g. 30 dB or a given fraction of its maximum transmit power level) These are particularly straightforward techniques to use. If desired, the amount of reduction could be selected randomly from a suitable range of reduction amounts or lower power values. This may help to ensure that e.g. mobile units, signals arrive at a base station over a broader range of values, thereby, as discussed above, helping the aloha process. More sophisticated methods of deriving the lower power level could be used, such as deriving the desired level from a path loss calculation (e.g. using the method of the present invention or the open loop power control method), or by using the closed loop power control method.

Where desired, a plurality of threshold levels could be used, and a different defined power level set for each threshold. For example, a first highest threshold could trigger transmission at a first lower defined power level, but if the received signal falls below a second lower threshold level, a higher defined power level could be used and so on. This avoids immediately transmitting at the maximum possible power level (which e.g. may be undesirable for battery economy) where it may not be entirely necessary to do SO. The threshold level(s) for the reverse change could be set to different values to provide some hysteresis.

The method and apparatus of the present invention can be used as desired to control the transmit power level of a radio unit. It could, for example, be used to control the transmit power level at all times. Where it is being used to control the transmit power level of a mobile radio unit, it is preferably used to control at least the transmit power of a mobile radio unit's initial access transmission to a base station, although thereafter preferably other forms of power control, such as open loop or closed loop power control are used to allow the unit to operate at a lower power level for reasons of e.g. battery economy.

In a preferred embodiment, the transmit power level calculation of the present invention is carried out periodically, preferably at predetermined regular intervals, to enable the mobile radio unit to be ready at any time to transmit at an appropriate power level.

In another embodiment of the present invention, the base stations of the radio system, could be controlled to switch to transmit at a higher power level if their receiver is experiencing overload from a mobile unit's signal, i.e. a mobile unit's signal is being received at above the maximum permitted signal level. If the base station raises its transmitter power level, the overloading mobile radio unit operating in accordance with the present invention will be induced to reduce its own output power level (because it will, e.g., calculate a lower path loss). This embodiment therefore provides a method of further controlling the power of mobile radio units close to a base station.

In this embodiment the mobile radio unit preferably uses the peak received signal in a given, e.g.

predetermined, time period, e.g. 60 seconds, instead of the averaged RSSI, as the estimated received signal power, as that means that it is only necessary for the base station to transmit at higher power briefly.

The base stations could also periodically transmit their actual transmitted power levels to the mobile radio units to allow the mobile radio units to perform additionally more accurate path loss calculations, e.g.

in an open loop power control manner, and/or to update stored values if desired.

The means for carrying out methods in accordance with the present invention may comprise pure hardware means such as discrete components or hard-wired logic gates. Alternatively, the methods may be implemented at least partially using software e.g. computer programs.

It will thus be seen that when viewed from a further aspect the present invention provides computer software specifically adapted to carry out the methods hereinabove described when installed on data processing means, and a computer program element comprising computer software code portions for performing the methods hereinabove described when the program element is run on a computer. The invention also extends to a computer software carrier comprising such software which when used to operate a mobile radio system comprising a digital computer causes in conjunction with said computer said system to carry out the steps of the method of the present invention. Such a computer software carrier could be physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like.

It will further be appreciated that not all steps of the method of the invention need be carried out by computer software and thus from a further broad aspect the present invention provides computer software and such software installed on a computer software carrier for carrying out at least one of the steps of the methods set out hereinabove.

The present invention is, for example, applicable to any suitable mobile radio communications system where the transmit power of mobile radio units of the system can be controlled, such as cellular telephone systems, 24 such as GSM (Global System for Mobile Communication) and private mobile radio systems, such as TETRA.

It can be seen that in the present invention, particularly in its preferred embodiments, an advantageous power control mechanism is provided that avoids excessive compression of the range of signal powers received by a base station, thereby reducing the damage to the aloha access mechanism caused by the power control. In addition there is no need to apply additional compression to the power levels received at the base station to allow for the effects of differential Rayleigh fading on the mobile radio unit's path loss calculations. (In open loop power control, as discussed above, power levels are typically limited to just above the base station receiver's limiting sensitivity. When working at this level, it is quite possible that a 20dB fade will be encountered on the uplink path. If the level at which the base station requests to receive signals is not uplifted by at least this much the mobile radio unit will be rendered unreceivable for the duration of the fade. In addition, the mobile radio unit may experience a downlink signal peak while making its downlink pathloss estimate. This could add to the error in the level received by the base station. In the present invention, at least in its preferred embodiment, fades (which are sharper and deeper than signal peaks) are not so much of a problem as the signals are intended to arrive at the base station at well above the limiting sensitivity. In addition, the present invention comes into effect at high signal levels where direct line of sight reception is probable. Fading only becomes deep when signals arrive at an antenna at equal levels from two or more directions with 180 deg phase error (destructive interference), most likely two different reflected signals. A direct signal will almost always be significantly stronger than reflected signal. Thus in - the present invention, it is not so necessary to make significant reductions in power level to ensure that downlink fades (for example) do not cause transmission at too high a power level, because at the high power levels at which the present invention will tend to operate, particularly in its preferred embodiments, the mobile radio unit is almost certainly in direct line of sight of the base station. Even if an error is made, the result will not be loss of signal, it will be temporary overload of the base station which is not so serious.) A number of preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figures 1 and 2 are schematic drawings of radio cells in a mobile radio communications system.

Figure 1 shows schematically a small radio cell 1 served by co-located, relatively low power base stations BS2 and BS3. Base station BS2 is connected to antenna A7, and base station BS3 is connected to antenna A8. (In an alternative implementation, BS2 and BS3 can share a single antenna, using isolators and circulators to separate them, but we show two antennae here for simplicity.) Mobile radio unit or station MS4 is very close to base stations BS2 and BS3 and is in danger of overloading their receivers if it were to transmit on full power. For example, if mobile station MS4 were to transmit to base station BS2 on a first RF channel, it might limit the ability of base station BS3 to receive a weak signal from distant mobile station MS5 on a second, adjacent RF channel. Also, for example, in a TDMA system, the strong signal received by base station BS2 on a first timeslot from mobile station MS4 might cause a large DC offset in the receiver which would be hard for the base station receiver to remove in time to receive a weak signal from another mobile station MS6 in the next timeslot (since this weak signal could be less than the DC offset caused by the large signal).

Figure 2 shows schematically a large radio cell 9 serviced by high power base stations BS10 and BS11, connected to antennae A14 and A15 respectively. Mobile station MS12 is at a moderate distance from base station BS10, but receives a strong signal from it via antenna A14. Mobile station MS13 is at the edge of the coverage of the system, and receives a weak signal from base station BS11 via antenna A15.

The operation of mobile station MS4 in Figure 1 in accordance with the present invention will now be described. Mobile station MS4 has a prestored predetermined limit of -50 dBW which is pre-assigned and pre-selected for the reception of mobile station signals by the plural base stations of the system (this is the maximum specified input signal for a TETRA base station, for example). Mobile station MS4 has stored as a predetermined transmit power level for all the plural base stations of the system a pre-assigned and preselected value of -2 dBW (0.6 Watt; this is the minimum specified output power of a TETRA base station).

Suppose mobile station MS4 detects a signal level of -48 dBW from base station BS1 averaged over 1 second. Mobile station MS4 will then calculate the averaged path loss from base station BS2 to mobile station MS4 as being 46 dB. (It in effect makes the assumption that it is receiving from a low power base station; in this example the assumption is correct.) Mobile station MS4 accordingly adjusts its output power for its initial access transmission to base station BS2 to -4 dBW, so that its transmissions will be expected to reach base station BS2 at the maximum permitted level, but cannot reach base station BS2 and base station BS3 at a level exceeding the pre- stored limit of -50 dBW (assuming its transmitter power setting is accurate and the uplink 27 - path loss is equal to the average downlink path loss) Mobile station MS4 could subtract a fade margin from its output power to take account of the possibility that it might be stationary (in which case the averaging of the RSSI gives no benefit) in a downlink loss maximum (a fade) and an uplink loss minimum. However, if mobile station MS4's assumption of being close to base station BS2 is correct, it is unlikely that there will be significant differences in the uplink and downlink path losses, and an error margin of 3 dB, for example (to cover transmitter power errors) will suffice. Of course in practice other error margins can be used. For example under some circumstances the Applicants believe that an error margin of up to 40 dB may be appropriate.

In this preferred case, mobile station MS4 therefore sets its output power to -7 dBW, ensuring its reception at base stations BS2 and BS3 in the range -56 dBW to -50 dBW.

once it has accessed base station BS2, mobile station MS4 can continue to transmit to base station BS2 (e.g. during the subsequent call) at its derived output power level of -7 dBW. Alternatively, it can control its power level for subsequent transmissions to a different, e.g. lower, level e.g. in accordance with closed loop power control messages received from base station BS2.

Consider now the operation of mobile station MS12 in Figure 2. It measures an averaged received signal level of -48 dBW from base station BS10 via antenna A14.

Mobile station MS12 again uses a stored predetermined base station power level of -2 dBW for its subsequent calculations (i.e. it assumes, incorrectly in this case, that it is receiving f rom a low power base station). As before, mobile station MS14 computes an 46 dB path loss, subtracts a 3 dB error margin (for example) and sets its output power to 7 dBW. (Again this power level can be used for the initial access transmission and then a 28 - different power level used for the subsequent call, if desired.) Consider that the actual output power of base station BS10 was 14 dBW, so the true path loss was 62 dB. As a result, the signal from mobile station MS12 reaches base station BS10 at a power level in the range -69+/-3 dBW if the losses on the uplink and downlink are identical. (A difference in the path loss could result in the base station receiving a signal perhaps 20 dB weaker or stronger than indicated.) However, since these signals are well above the limiting sensitivity, of about -140 dBW, of the base station, the error in mobile station MS12's calculations will not result in loss of signal.

is In a mobile radio communications system, a mobile station would typically be limited to a maximum power of, e.g. 10 dBW. As can be seen, such a mobile station operating in accordance with the embodiment of the present invention as described above, would transmit at its maximum power until it were so close to the base station that the path loss fell below 63 dB (allowing for a 3 dB error margin) for a low power base station, or perhaps 79 dB (allowing for a 3 dB error margin) for a high power base station. Thus, in practice, the mobile station's output power will only be reduced in a small area 16 at the centre of the cell. otherwise, regardless of its distance from the base station, the mobile radio unit will transmit at its maximum power level. In this way, the compression of the power range received by the base station is reduced.

It can be seen that the present invention can restrict the peak power received by the base stations, but particularly in its preferred embodiment, does not strongly equalize the power level of all received signals. It compresses the range of signals just enough to limit the effects of signal overload on the base station receivers. This is less damaging to aloha random access mechanisms than attempting to achieve equality of all signals at a relatively low level.

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Claims

1. A method of controlling the power level at which a first radio unit of a radio communications system transmits to another radio unit of the system, the method comprising:

determining the power level received at the first radio unit of a radio signal from the other radio unit; using the determined received power level, a first stored predetermined power level value representative of the transmit power level of the other radio unit, and a second power level value representative of a desired received power level at the other radio unit, to derive a transmit power level for the first radio unit; and controlling the first radio unit to transmit signals to the other radio unit at a transmitted power level which is the lower of the derived transmit power level and a defined maximum transmit power level of the first radio unit.

2. The method of claim 1, wherein the first radio unit is a mobile radio unit of a mobile radio communications system and the other radio unit is a base station or another mobile radio unit of the system.

3. The method of claim 1 or 2, wherein the derived transmit power level for the first radio unit is derived by: estimating the path loss for the signal from the other radio unit by determining the difference between the stored first predetermined power level value representative of the transmit power level of the other radio unit and the determined received power level; and summing the estimated path loss with the second power level value representative of the desired received power level at the other radio unit to derive the transmit power level.

4. The method of claim 1, 2, 3 or 4, wherein the defined maximum transmit power level of the first radio unit varies depending on the location of the radio unit.

5. The method of any one of claims 1 to 4, wherein the first stored predetermined transmit power level value representative of the transmit power level of the other radio unit is based on the lowest possible transmit power level usable in the radio system by any radio unit of the type of the other radio unit.

6. A method of controlling the power level at which a radio unit of a radio communications system transmits to another radio unit of the system, the method comprising:

is determining the power level received at the radio unit of a radio signal from the other radio unit; using the determined received power level, a first stored predetermined power level value corresponding to a predetermined minimum possible transmit power level of the other radio unit of the system, and a second power level value representative of a desired received power level at the other radio unit, to derive a transmit power level for the radio unit; and controlling the radio unit to transmit signals to the other radio unit at a transmitted power level which is the lower of the derived transmit power level and a defined maximum transmit power level for the radio unit.

7. The method of any one of claims 1 to 6, wherein the first and second power level values and the derived transmit power level derivation process are arranged so as to derive for the derived transmit power level, the power level considered necessary by the transmitting radio unit for its signal to arrive at the other radio unit at a level equal to, or at or within a predetermined margin below, the maximum received signal level permitted at the other radio unit in the system for signals from a radio unit of the type of the first radio unit.

8. The method of any one of claims 1 to 7, wherein the second power level value representative of the desired received power level at the other radio unit is based on the maximum power level at which the radio system permits signals to be received at that radio unit.

9. A method of controlling the power level at which a mobile radio unit of a mobile radio communications system transmits to a base station of the system, comprising:

deriving a transmit power level for the mobile radio unit on the basis of the received power level of a signal from a base station of the system and a defined maximum permitted received signal level at the -base station; and controlling the mobile radio unit to transmit to the base station at the lower of the derived transmit power level and a defined maximum transmit power level for the mobile radio unit; wherein the derived transmit power level is the power level which is estimated to be necessary for a transmission from the mobile radio unit to arrive at the base station at a signal level which is equal to, or at or within a predetermined margin below, the defined maximum permitted received signal level.

10. The method of claim 9, wherein the derived transmit power level is derived by:

estimating the path loss to the base station by subtracting the received power level of the base station's signal from a predetermined power level value representing the transmit power level of the base station, and then adding the estimated path loss to the maximum permitted received signal level at the base station.

11. The method of any one of claims 1 to 10, wherein a transmit power level for the first, transmitting radio unit is only derived using the stored predetermined power level values if the estimated received signal power from the other radio unit is above a predetermined threshold level; and if the estimated received signal power is below the threshold level, the radio unit then uses the defined maximum transmit power level.

12. A method of controlling the power level at which a first radio unit of a radio communications system transmits to another radio unit of the system, the method comprising:

estimating the received power level at the first radio unit of a signal from another radio unit of the radio communications system; determining whether the received power level is above a predetermined threshold level; if the received power level is above the predetermined threshold level, deriving a transmit power level for the first radio unit based on the received power level; and if the received power level is not above the predetermined threshold level controlling the radio unit to transmit signals to the other radio unit at a defined transmit power level.

13. A method of operating a radio unit of a radio communications system, the method comprising:

14. The method of claim 13, wherein if the received power level is above the predetermined threshold level, the radio unit is controlled to use a predetermined power level that is less than its defined maximum level, or to reduce its current transmit power by a predetermined amount or by an amount selected at random from a range of reduction amounts or of lower power level values.

15. The method of claim 13 or 14, wherein there are plural predetermined threshold levels, and a different power level or power range is set for each threshold.

16. An apparatus for controlling a radio unit of a radio communications system, the apparatus comprising:

17. An apparatus for controlling a mobile radio unit of a mobile radio communications system, the apparatus comprising:

means for storing a first predetermined power level value representative of the transmit power level of a base station of the mobile radio communications system; means for storing a second power level value representative of a desired received power level of a base station of the mobile radio communications system; means for estimating the power level received at the mobile radio unit of a radio signal from a base station of the mobile radio communications system; means for using the estimated received power level, and the first and second stored power level values, to derive a transmit power level for the mobile radio unit; means for comparing the derived transmit power level and a defined maximum transmit power level of the mobile radio unit; and means for controlling the mobile radio unit to transmit its signals to the base station at the lower of the derived transmit power level and the defined maximum transmit power level for the mobile radio unit.

18. An apparatus for controlling a first mobile radio unit of a mobile radio communications system, the apparatus comprising:

means for storing a first predetermined power level value representative of the transmit power level of a mobile radio unit of the mobile radio communications system; means for storing a second power level value representative of a desired received power level of a mobile radio unit of the mobile radio communications system; means for estimating the power level received at the first mobile radio unit of a radio signal from another mobile radio unit of the mobile radio communications system; means for using the estimated received power level, and the first and second stored power level values, to derive a transmit power level for the first mobile radio unit; means for comparing the derived transmit power level and a defined maximum transmit power level of the first mobile radio unit; and means for controlling the first mobile radio unit to transmit its signals to the other mobile radio unit at the lower of the derived transmit power level and the defined maximum transmit power level for the first mobile radio unit.

19. The apparatus of claim 16, 17, or 18, wherein the apparatus stores and uses the same single first predetermined power level value representative of the transmit power level for all radio units of the radio communications system of a particular type.

20. The apparatus of claim 19, wherein a single predetermined transmit power level value is stored as representing the transmit power level of all base stations of the radio system.

21. The apparatus of any one of claims 16 to 20, wherein the first stored predetermined transmit power level value representative of the transmit power level of the other radio unit is based on the lowest possible transmit power level usable in the radio system by any radio unit of the type of the other radio unit.

22. The apparatus of claim 21, wherein, where the radio system is a mobile radio communications system and the other radio unit is a base station of that system, the stored predetermined base station transmit power level value is based on the lowest possible transmit power level usable by any base station of the system.

23. The apparatus of any one of claims 16 to 22, wherein the apparatus stores and uses the same predetermined second power level representative of the desired received power level at the other radio unit for all radio units of the radio communications system of a particular type.

24. The apparatus of claim 23, wherein a single predetermined received power level value is stored as representing the desired received power level at all base stations of the radio system.

25. The apparatus of any one of claims 16 to 24, wherein the radio communications system is a mobile radio communications system, and each mobile radio unit of the mobile radio communications system stores and uses the same predetermined values indicative of the transmit power of the base stations of the mobile radio communications system and stores and uses the same predetermined values indicative of the desired received power level at the base stations.

26. A mobile radio communications system comprising plural mobile radio units and plural base stations capable of radio communication with the mobile radio units, wherein each mobile radio unit comprises:

means for storing a first predetermined power level value to be used as representing the transmit power level of each base station of the mobile radio communications system; means for storing a second predetermined power level value to be used as representing the desired received power level of each base station of the mobile radio communications system; means for estimating the power level received at the mobile radio unit of a radio signal from a base station of the mobile radio communications system; means for using the estimated received power level, and the first and second stored power level values, to derive a transmit power level for the mobile radio unit; means for comparing the derived transmit power level and a defined maximum transmit power level for the mobile radio unit; and means for controlling the mobile radio unit to transmit its signals to the base station at the lower of the derived transmit power level and the defined maximum transmit power level for the mobile radio unit; wherein each mobile radio unit of the system stores and uses the same first and second predetermined power level values.

27. The apparatus of any one of claims 16 to 26, wherein base stations of the radio system comprise means for switching to transmit at a higher power level if their receiver is receiving a radio unit's signal at greater than a predetermined maximum permitted signal level.

28. The apparatus of any one of claims 16 to 27, further comprising:

means for determining if the estimated received signal power from the other radio unit is above a predetermined threshold level; means for deriving a transmit power level for the radio unit using the stored predetermined power level values if it is determined that the received signal power is above the predetermined threshold level; and means for controlling the radio unit to use the defined maximum transmit power level if the estimated received signal power is below the threshold level.

29. A radio unit for use in a radio communications system, the radio unit comprising:

means for estimating the received power level at the radio. unit of a signal from another radio unit of the radio communications system; means for determining whether the received power level is above a predetermined threshold level; means for, if the received power level is above the predetermined threshold level, deriving a transmit power level for the radio unit based on the received power level; and means for, if the received power level is not above the predetermined threshold level, controlling the radio unit to transmit signals to the other radio unit at a defined transmit power level.

30. A mobile radio unit for use in a mobile radio communications system, the mobile radio unit comprising:

means for estimating the received power level at the mobile radio unit of a signal f rom a base station of the mobile radio communications system; means f or determining whether the received power level is above a predetermined threshold level; means for, if the received power level is above the predetermined threshold level, controlling the mobile radio unit to transmit signals to the base station at a power level lower than a defined maximum transmit power level of the mobile radio unit; and means for, if the received power level is not above the predetermined threshold level, controlling the mobile radio unit to transmit signals to the base station at the defined maximum transmit power level of the mobile radio unit.

31. The radio unit of claim 30, wherein if the received power level is above the predetermined threshold level, the radio unit is controlled to use a predetermined power level that is less than its defined maximum level, or to reduce its current transmit power by a predetermined amount or by an amount selected at random from a range of reduction amounts or of lower power level values.

32. The radio unit of claim 30 or 31, wherein there are plural predetermined threshold levels, and a different power level or power range is set for each threshold.

33. A mobile radio unit of a mobile radio communications system comprising the apparatus of any one of claims 16 to 28.

34. Computer software specifically adapted to carry out the method of any one of claims 1 to 16 when installed on a computer.

35. A method of operating a radio unit of a radio communications system substantially as hereinbefore described with reference to any one of the accompanying drawings.

36. An apparatus for controlling a radio unit of a radio communications system substantially as hereinbefore described with reference to any one of the accompanying drawings.